The invention relates to a projection television display device comprising three monochrome projection television display tubes each having a display screen which is provided on the inside of the display window of the tube and luminesces in a different colour. Each tube has an optical axis extending perpendicularly from the centre of the display screen, the optical axes of the three display screens being co-planar and the axes of the display screens of the first and second tubes coinciding and the axis of the display window of the third tube constituting the main axis of the device, said axis being perpendicular to the two coincident axes. The device has two flat dichroic intersecting reflective mirrors extending perpendicularly of the plane and through the point of intersection of the axes and each constituting an angle of 45.degree. with the main axis. The first mirror reflects the image of the first display screen and the second mirror reflecting the image of the second display screen into the direction of the main axis and away from the third display screen, filters being disposed between the display screens and mirrors for passing the desired colour of light.
In this manner the monochrome images generated on the three display screens are combined and displayed as one colour image on the projection screen.
A projection television display device of this type is known from Japanese Patent Application 59-139537. In this device absorption filters (coloured glass) are used between the display screens and the mirrors in order to reduce colour errors and loss of contrast. In the Japanese Patent Application 59-139536 pigmented phosphors are used for the same purpose.
A projection television display device of this type without the filters has extensively been described in the article "A Single-Lens Three CRT Crossed Dichroic Color projector for Data and Video", Terry C. Schmidt, Electrohome Limited, Kitchener, Ontario, Canada, SID 85 Digest, P 270-3 which is hereby incorporated by reference.
In the German Patent Application 2330898 laid open to public inspection a display tube has been described in which a multilayer interference filter is used between the luminescent material (the phosphor) and the display window. For the layer thicknesses an optical thickness of less than 0.25.lambda. or between 0.5 and 0.75.lambda. is chosen for the layers of the filter consisting of a material having a high refractive index, with .lambda. being the wavelength of the light emitted by the luminescent material. For the layers of the filters consisting of material having a low refractive index an optical thickness of 0.25.lambda. or an odd multiple thereof is chosen. The filter changes the light radiation characteristic of the display screen of the tube in such a manner that the quantity of light radiated within a limited angular aperture increases by 25% as compared with a tube without a filter. The contrast in the generated image is also increased due to a reduction in background brightness.
British Patent Specification 1,306,335 describes a similar display tube in which a passband interference filter is disposed between the phosphor and the display window. This filter consists of layers having an optical thickness of 0.25.lambda. and has the following design: EQU S L H L L L L L L H L H L L L L L L H L H L L L L L L H
in which S is the display window, L denotes layer of a material having a low refractive index and H denotes layers of a material having a high refractive index. Such a filter may also be defined as follows: EQU S L H (L).sup.6 H L H (L).sup.6 H L H (L).sup.6 H
Due to the presence of the six mating layers having a low refractive index (L).sup.6 a Fabry-Perot filter is formed. For a given choice of the optical layer thickness this filter has a desired transmission band for light rays constituting an angle of less than 25.degree. to 35.degree. with the normal of the filter. In addition, however, there is an unwanted broad transmission band for light rays constituting an angle of between 55.degree. to 90.degree. with the normal. All the light passed in this band is lost or contributes to what is referred to as the halo and thereby to a loss of contrast in the displayed image. This will be further explained hereinafter. Moreover, the cryolite layers used in this filter and hygroscopic and remain soft so that the filter may easily be damaged. The zinc sulphide (ZnS) layers used in this filter easily oxidise when firing the tube at approximately 475.degree. C. so that they are less suitable for use in television display tubes.
German Patent Application 3440 173 laid open to public inspection describes a projection television display tube having a six-layer interference filter between the phosphor and the display window. When such a filter is used, light is also radiated at relatively large angles with the normal on the display screen, as is apparent from the Figures. Projection television display tubes having interference filters have also between described in the non-prepublished prior Netherlands Patent Application 8402034 to which U.S. Pat. No. 4,634,926 corresponds, hereby incorporated by reference, and the non-prepublished British Patent Application 8513558 to which U.S. Pat. Nos. 4,683,398 corresponds hereby incorporated by reference.